(a) Schematic of a metasurface enables unidirectional SPP excitation. Near-field scanning optical microscopy image shows left circular polarized light CPL is coupled to SPPs propagating to the right (b), while right circular polarized light is coupled to the SPPs propagating to the left (c). Reprinted from ref. [65] with permission. (d) Schematic of a unidirectional SPPs coupler. (e) Experimental result shows SPPs are only excited to the right direction for an incident beam with right circular polarization. (f) Experimental result shows SPPs are only excited to the left direction for an incident beam with left circular polarization. Reprinted from reference [66] with permission.